Sisters Country Geologic History

About 280 million years ago, all the continents were assembled together into a supercontinent--Pangea. Present day “continental" Oregon did not exist. About 170 million years ago, Pangea broke apart along boundaries resembling present day continental margins. The break-up started with a spreading rift system (red dashed line) forcing North America to move northwest and creating the Atlantic Ocean as it widened. The splitting-up of Pangea motion initiated the coastal subduction zone that strongly influences Oregon geology. The blue dotted line shows the ancient shoreline of western North America at this time.

As Pangea broke-up and spread apart, the North American continent overrode the Pacific Ocean floor and created a SUBDUCTION ZONE along the Oregon coast (and other places). The continental material is lighter and tends to “float” on the heavier basaltic oceanic material. At about 60 miles in depth, the down-going slab melts and sends plumes of molten rock (magma) to the surface. In places, these bodies of magma emerge as volcanoes and other types of volcanic eruptions.

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